An amplifier is a circuit that has many uses in electronics. The most important application of an amplifier is to change, and particularly to increase, the voltage level or the current intensity level of an electrical signal. Occasionally an amplifier is also used to isolate an amplifier input signal from the output, e.g. to prevent sources of noise from feeding back into the signal source.
Under most circumstances, it is desirable that the amplifier degrades the signal as little as possible. Undesirable degradation particularly includes noise and distortion.
At the same time, it is important for many applications that the amplifier""s power consumption be as small as possible. A compromise must be found between these requirements when designing an amplifier, because they cannot be optimized mutually. Non-linear distortions particularly increase significantly as the amplifier design tends to favor reduced power consumption. This relationship is true in general, but is particularly marked in amplifiers that are manufactured in CMOS technology. As a result, many products can only be produced in CMOS technology if the performance is reduced or the power consumption is increased.
In applications for which the prime concern is to achieve a signal as free from distortion as possible, countercoupled amplifier circuits are used. FIG. 7 shows an example of a countercoupled operational amplifier OPV, which amplifies a circuit input signal Vin in conventional manner to create a circuit output signal Vout.
As is illustrated in FIG. 8, a good approximation of the operational amplifier OPV can be achieved with a serial connection from an ideal amplifier having frequency-independent amplification V, a low-pass filter TP(f) and a non-linear voltage transmission function NL(Vm), where f is the signal frequency and Vm is the voltage supplied to the output stage of the operational amplifier. This model is predicated on the assumption that the output stage of amplifier OPV is the dominant source of the non-linear distortion. A qualitative representation of the undesirable non-linearity NL(Vm) is shown in FIG. 9.
The non-linearity NL(Vm) is determined by the non-linear voltage-to-current characteristic of the amplifier transistors and the current that is flowing through these transistors (xe2x80x9ctransistor non-linearityxe2x80x9d). The smaller this current is, the more marked is the non-linearity. This is the case, for instance, when the amplifier is rated to operate with a relatively small current.
The effect of the non-linearity of amplifier OPV on the signal can be reduced by countercoupling amplifier OPV, as is shown in FIG. 7. This reduction is proportional to the loop amplification Aloop(f):
Aloop(f)=V*TP(f)*R1/(R1+R2) 
In order to obtain a high degree of linearity of the signal, it is necessary to aim for a high loop amplification Aloop(f) in the frequency range under consideration. To this end, the following two methods are used:
1. Use of a high transit frequency of the amplifier loop
The transit frequency ftransit is the frequency at which the loop amplification Aloop(f) is reduced to value 1 due to the effect of the low pass filter TP(f), i.e. Aloop(ftransit)=1. The higher the transit frequency ftransit is in comparison to the signal frequencies f, the lower the reduction of the loop amplification Aloop that is caused by low-pass filter TP. Amplifiers that use a current negative feedback (xe2x80x9ccurrent feedback amplifierxe2x80x9d) achieve a high transit frequency.
If the desired high linearity is obtained by means of a high transit frequency ftransit, the power consumption of the amplifier is also high. It is not possible to achieve low power consumption and high transit frequency using the transistors that are currently available. The present method is therefore appropriate only for applications in which power consumption is of lesser importance.
2. Use of multiple amplification stages connected in series
In this method, the amplification at frequencies f below the transit frequency ftransit is raised instead of the transit frequency ftransit. This is achieved with the use of a low pass filter of a higher order. This method allows for low power consumption with high linearity of the amplifier circuit, provided signal frequencies f are sufficiently low. Amplifiers of this kind are designated, for example, by the names xe2x80x9cNested Millerxe2x80x9d and xe2x80x9cDouble Nested Millerxe2x80x9d.
The advantage of having amplification stages connected in series becomes less evident as the signal frequencies f approach the transit frequency ftransit, as is shown in the following example:
With the architecture shown in FIG. 7, a sinusoidal signal having frequency f=10 MHz is to be amplified by 20 decibels (dB). Let the operational amplifier have a transit frequency of ftransit,amp=990 MHz and let the feedback have an impedance ratio R1/R2={fraction (1/10)}. The transit frequency of the loop amplification ftransit,loop is then:                               f                      transit            ,            loop                          =                  xe2x80x83                ⁢                              f                          transit              ,              amp                                *                      R1            /                          (                              R1                +                R2                            )                                                              =                  xe2x80x83                ⁢                              f                          transit              ,              amp                                *                      R1            /                          (                              R1                +                                  10                  *                  R1                                            )                                                              =                  xe2x80x83                ⁢                  990          ⁢                      xe2x80x83                    ⁢          MHz          *                      1            /            11                                                  =                  xe2x80x83                ⁢                  90          ⁢                      xe2x80x83                    ⁢          MHz                    
In a two-stage operational amplifier OPV with a first order low-pass filter, the amplification in this frequency range is then close to:
Aloop(f)=ftransit,loop/f 
It follows that, for the third harmonic (f3) of signal frequency f, which has a frequency of 30 MHz, the loop amplification Aloop(f3) is:
Aloop(f3)=ftransit,loop/f3=ftransit,loop/(3*10 MHz)=90 MHz/30 MHz=3=9.54 dB 
The non-linearity of the output stage at this frequency f3 is therefore reduced by 9.54 dB. If the application requires, for instance, a signal-to-distortion ratio (S/D) of 70 dB up to 30 MHz, the base linearity of the output stage must be at least 60.46 dB. This can only be achieved with very high currents (class A output stages).
If a three-stage operational amplifier (xe2x80x9cNested Millerxe2x80x9d amplifier) is used, loop amplification Aloop is increased by about 3 dB at 30 MHz. The requirement for linearity of the output stage is therefore reduced to 57.46 dB. If more amplification stages are added, the effect is negligible ( less than 1 dB). This is because, for reasons of frequency compensation, each additional amplification stage must be slower by one third than the stage to which it is connected. If signals in the MHz frequency range with high linearity requirements have to be amplified, up to now this can only be achieved with a correspondingly high transit frequency and high output stage linearity. However, high output stage linearity and high transit frequency both lead to high power consumption.
The present invention treats in the first instance of an amplification circuit having a circuit input for a circuit input signal and an amplification zone for amplifying the circuit input signals. The present invention treats in the second instance of a process for amplifying a signal.
The object of the present invention is to provide an amplification circuit and a process for signal amplification entailing reduced power consumption, with which signal distortions can largely be precluded.
The amplification circuit according to the invention is characterized in that the amplification zone includes two amplifiers, each of which is countercoupled, and to which the circuit input signal is fed in parallel, and the amplifier outputs of which are or can be connected with a circuit output to produce a circuit output signal, wherein the amplification input zone of one of the two amplifiers is connected with the amplification input zone of the other of the two amplifiers by means of a further amplifier.
The process for signal amplification according to the invention is characterized in that the signal is amplified in parallel through two countercoupled amplifiers and the two amplifier output signals are or can be combined to produce the amplified signal, wherein a signal that has been split from the amplification input zone of one of the two amplifiers is amplified and supplied to the amplification input zone of the other of the two amplifiers.
The combination of the amplification output signals via their respective loads or working resistances (e.g. resistors, capacitors, etc.) represents a weighted addition of the amplifier output signals.
The invention further provides that as a result of this arrangement, signal distortions of the two amplifier output signals largely cancel each other out in the circuit output signal.
The basic idea of the invention consists in that, for example, for a signal having a AC component, the signal is amplified in parallel on two separate signal paths that are arranged in such manner that the two amplified signals are combined in phase at the circuit output, while the distortions are combined out of phase and preferably in opposing phase relationship. When the amplifier output signals are added at the circuit output, the wanted signals are added, but the distortions more or less cancel each other out (the signal is already improved if the distortions in two combined amplifier output signals cancel each other out at least in part).
By judicious selection of the properties of the individual amplifiers, in particular the amplification factor of the additional amplifier, it is possible to achieve almost complete cancellation of the distortion components. At the same time, the nature and magnitude of the distortions in the two amplification paths is unimportant, provided that the distortions are largely opposed to each other on the output side.
With this invention it is possible to provide amplification circuits in which the signal distortion of the circuit output signal is smaller than the signal distortions in the two amplifier output signals over wide signal frequency ranges. This applies particularly for higher portion of the frequency range as determined by product specification, which is of particular interest in practical applications.
The amplification according to the invention enables signal distortion to be largely eliminated. In addition, since the amplifiers of the circuit according to the invention can both be created considerably smaller (xe2x80x9cscaledxe2x80x9d) than a conventional amplifier having comparable characteristics, the signal amplification according to the invention is associated with markedly lower power consumption in comparison thereto. Particularly, the individual load resistances provided for combining the signals can also be scaled, while the overall working resistance is unchanged. This scaling of associated components is particularly easy to achieve in integrated circuits on a chip if these loads are integrated with the amplifiers in question on the chip.
With the present invention, the power consumption of the amplifier circuit can thus be reduced to a minimum while largely eliminating signal distortions.
In a preferred embodiment of the invention, the two amplifiers have essentially the same configuration, but the size of the second amplifier differs from that of the first amplifier in accordance with a relative scaling factor. In this way, two amplification paths with differing signal weighting factors, but otherwise very similar characteristics are created in a very simple manner.
In a further embodiment of the invention, the two amplifiers have essentially the same configuration, but the size of the second amplifier and its working resistance differs from that of the first amplifier in accordance with a relative scaling factor s. For effective elimination of distortion in this embodiment, which is especially preferred for integrated circuits, amplification A of the additional amplifier and the relative scaling factor s should approximately satisfy the following condition: A=1/s+1. In this case, the distortions at the circuit output cancel each other out almost totally. The additional amplifier A should deviate from the value returned by this equation preferably by less than 20%, and more preferably by less than 10%.
In a further embodiment of the invention, the two amplifiers have essentially identical amplification properties (particularly amplification factor, distortion and working resistance) and the additional amplifier has an amplification of about 2. In this case too, the distortions at the circuit output cancel each other out almost completely. This embodiment can be realized quite simply with two identical amplifiers, each of which may be conceived, e.g. as amplifiers that have been scaled down by 50% (relative scaling factor s=1).
With regard to scaling, it is important to note the following: with modem production technologies, particularly CMOS technologies it is possible to produce transistors on a very small scale. Amplifiers almost always use transistors having dimensions that are considerably larger than the lower limit determined by production technology. In consequence, scaling such amplifiers represents no difficulties. This means, for example: if the driven working resistance of the amplifier is to be halved (=the loaded impedance is doubled), then all transistor parameters and all currents can be halved. The feedback network (cf. for instance R1, R2 in FIG. 7) can be scaled by the same factor, so that the input impedance of the overall amplification circuit is doubled. This scaling has no effect on the signal distortion caused by the amplifier. If two amplifiers, each of which has been scaled down by a factor of 50%, are connected in parallel, the overall behavior reflects that of the original unscaled amplifier, even with respect to input impedance and noise.
In a preferred embodiment of the invention, the amplification zone consists of two essentially identical amplifiers, the input zones of which are connected to one another by an additional amplifier having amplification 2. Alternatively, it is also possible to conceive of an arrangement involving further amplifiers arranged in parallel.
Amplifiers, particularly operational amplifiers, normally have a multistage configuration. An extension of the invention provides for integrating the additional amplifier in an amplification stage of the other of the two amplifiers, particularly in an input stage thereof.
Some degree of signal delay is associated with the additional amplification, and in the case of signals that vary with time, this impairs the distortion components elimination function on the output side. In order to improve the precision of the amplification, a further embodiment therefore provides for compensating for this signal delay or phase delay. Compensation of such kind is preferably integrated into the feedback path of at least one of the two amplifiers. If the two amplifiers are not identical, this compensation means may also take into account differing signal transit times of the two amplifiers.
The invention described in the foregoing allows hitherto unattainable power-efficiency in amplifiers for working resistances which can be simultaneously driven from a multiplicity of different amplifier outputs. One particularly advantageous application of the invention is, for example amplification of a control signal (xe2x80x9cline driverxe2x80x9d).
In the circuit according to the invention, particularly the output stages of the individual amplifiers can be designed for very low power consumption, since their linearity requirements are reduced by orders of magnitude. Where previously class A amplifiers were required, this invention enables the use of classes AB or B. The economy in terms of power is then over 50% for the same total linearity. This is especially advantageous in respect of battery-powered devices. But reduced power consumption is also important for fixed-location use, since in many cases the integration density of integrated circuits cannot be increased because of the heat generated. This problem can be solved by the invention described in the foregoing.